1. Field of the Invention
This invention relates to well logging apparatus and more particularly the monitoring of depth information for well-logging operations.
2. Description of the Related Art
Zemanek, U.S. Pat. No. 3,369,626 discloses an ultrasonic tool for use in scanning the inner surface of an open well or of casing in a borehole. The tool, which is commercially known as the "borehole televiewer" creates a high resolution picture of the inner surface under investigation. The borehole televiewer is used to "see" the inner surface under investigation through drilling mud or other borehole fluids. In an open borehole, the borehole televiewer provides a picture of the formations surrounding the borehole. In a cased borehole, the borehole televiewer provides a picture of the inner surface of the casing, which can be used to determine the condition of the inner surface.
The borehole televiewer uses a rotating ultrasonic transducer, the transducer has a transmitter, to generate acoustic waveforms, and a receiver, to receive the acoustic return. The acoustic return is caused by the reflection of the generated acoustic waveform from the inner surface under investigation.
The acoustic return has two measured parameters, the time of travel of the acoustic return and the amplitude, which give an indication of the condition of the investigated surface.
The transducer rotates three revolutions per second, is pulsed about 500 times per revolution and is pulled up the borehole at a speed of about 5 feet per minute. The transducer spot size, the rotational speed, the pulse repetition rate and the vertical speed combined to provide full coverage of the investigated inner surface, resulting in high aereal resolution of the inner surface. In the past, depth was correlated with televiewer information by a voice entry on an audio track of a video cassette recorder (VCR). Depth was constantly monitored by the operator and verbally entered every five feet. This type of operation, however, is very tedious for an operator because the boreholes under investigation tend to be very deep and the logging rate is very low. The operator fatigue inherent in this evolution results in human errors being made in the depth correlations.
There is a need, therefore, for a simple and accurate borehole televiewer depth monitoring and recording system which can relieve the operator of this time consuming and tedious task. In addition, there is also a need for a depth monitoring and recording system which provides precise logging rate information at slow logging speeds.